Regarding a conventional in-wheel motor drive system, Japanese Patent Laying-Open Nos. 07-81430, 2000-343920, 2001-315534 and 11-170831 for example disclose that a motor is housed in an outer frame and an output shaft of the motor is rotatably supported on the outer frame by a bearing unit. One end of the output shaft is connected via a planetary gear to a wheel.
The outer frame that houses the motor is connected via a ball joint to a suspension arm. The suspension arm is connected via a shock absorber to a body of a vehicle (Japanese Patent Laying-Open No. 07-81430).
As seen from the above, in the conventional in-wheel motor system, the outer frame that houses the motor is connected to the vehicle's body via the ball joint and the suspension arm.
Further, from “Development of Dynamic-Damper Type In-Wheel Motor” by Goh Nagaya, Yasumichi Wakao and Akihiko Abe, Proceedings of Annual Congress, Society of Automotive Engineers of Japan, Inc., No. 83-02, pp. 9-12, Nov. 26, 2002, a conventional in-wheel motor drive system is known according to which a hollow motor is supported by a motor suspension. The hollow motor is connected to a wheel to rotate it. The hollow motor is supported by the motor suspension so that the hollow motor can vibrate in the top-bottom direction of the vehicle and is accordingly isolated from the unsprung weight. The wheel is supported on the vehicle by a suspension arm. Regarding this in-wheel motor drive system, when the wheel unit (including the tire) vibrates, the hollow motor receives the vibrations of the wheel unit via the wheel to vibrate in the top-bottom direction of the vehicle. The vibrations of the hollow motor cancel vibrations of the unsprung part of the vehicle.
The conventional in-wheel motor drive system, however, encounters a problem as described below. When the wheel is displaced due to road surface conditions for example, the motor is also displaced so that there arises an unsprung input to the vehicle's body via the ball joint and the suspension arm, resulting in deterioration in ride comfort of the vehicle.